Cited as a means for obtaining a radiographic image for medical diagnosis or non-destructive testing of a various types of tissues and applying it to diagnosis and radiographic flaw detection are: radiography by a combination of a silver halide photographic light sensitive material and a radiographic intensifying screen, or radiographic image conversion method by the use of a stimulable phosphor from which, after absorption of radiation energy, the accumulated radiation energy is emitted in the form of fluorescence by stimulation with electromagnetic waves such as visible light or infra-red rays (hereinafter referred to as stimulable phosphor).
Diagnosis or examination with radiography is such that radiation transmitted through or emitted from photographic object is converted, through absorption by phosphor contained in the radiographic intensifying screen and its excitation, into visible light, which produces a radiographic image on the silver halide photographic light sensitive material. The radiographic image is formed by exposing, to radiation through an object, the silver halide photographic light sensitive material having on one side or both sides of a support a silver halide emulsion layer, which is in contact with a radiographic intensifying screen to a radiation through an object.
The phosphor has a high brightness and can form a radiographic image with a relatively small dose of radiation, so that exposure to radiation of the object can be minimized. It is well known that sharpness and graininess of the image depend upon the particle size and dispersion of the phosphor, and homogeneity and in particular upon the filling ratio in a phosphor containing layer.
The radiographic image conversion method employing the stimulable phosphor includes the employment of a radiation image converting panel containing the stimulable phosphor (hereinafter, referred to as stimulable phosphor panel). In this case, the radiation transmitted through or emitted from the object is absorbed by a stimulable phosphor contained in the panel, followed by stimulating time-sequentially the phosphor with electromagnetic waves such as visible light and infra-red rays (also known as stimulating light), and emitting the radiation energy accumulated in the phosphor, in the form of light (photo-stimulated luminescence). The photo-stimulated luminescence is read as electric signals and based the electric signals obtained, the object or its radiographic image is reproduced as a visible image. The panel which has already been read is treated to eliminate residual images and made ready for the next photograph. Thus, the conversion panel can be repeatedly employed.
Similarly to the screen, brightness, image sharpness and granularity of the panel are also dependent upon the particle size, dispersibility, homogeneity and filling ratio of the stimulable phosphor. Of these, the filling ratio of the stimulable phosphor is particularly affectional.
There was proposed a technique of enhancing the filling ratio by compressing the phosphor containing layer or stimulable phosphor containing layer. The phosphor or stimulable phosphor has a disadvantage in that it is liable to produce defect or destruction of the crystal structure under pressure, resulting in lowering of the sensitivity. In order to keep the phosphor or stimulable phosphor from destruction, it must contain a large amount of resin, so that the proportion of the resin in the phosphor layer or stimulable phosphor layer increases and light is more easily diffused, leading to undesired lowering of the sharpness.
JP-A 3-196036 (herein, the expression "JP-A" refers to an unexamined and published Japanese Patent Application) discloses a technique of improving sharpness of the screen by controlling the weight ratio of the binder and the filling ratio of the phosphor, in which the weight ratio of the binder to the phosphor and the filling ratio of the phosphor are defined to be 4-11% and 60-70%, respectively.
The upper limit of the filling ratio of the phosphor being 70% is ascribed to a higher ratio of the weight ratio of the binder and the fact that an increase of the phosphor filling ratio rapidly reduces the void ratio by volume as a factor of light scattering and as a result, light is easily diffused, resulting in deterioration of image sharpness.
JP-A 3-196036 described above discloses a technique of enhancing the filling ratio by applying compression-heating. In this technique, the more the filling ratio is enhanced, the more severe the compression-heating condition is forced to be, causing destruction of the phosphor and denaturation of the binder and leading to lowering of the sensitivity. In view thereof, it can be understood that the phosphor filling ratio is at most, 70%.
Contrarily, if the weight ratio of the binder is decreased without applying a means such as compression-heating or with heating under low compression, the phosphor filling ratio can easily exceed its upper limit of 70%.
As described in JP-A 3-196036, when the weight ratio of the binder is less than 4%, strength of the intensifying screen is often markedly lowered; therefore, and further modification is required for maintaining the strength of the screen at a desired level, along with a lower weight ratio of the binder.
Inventors of the present invention found that an intensifying screen with sufficient strength even when the binder weight ratio was 0.1 to 3.0%, having a high filling ratio of the phosphor even without being compressed or with a little compression, and capable of providing an image with superior performance, could be obtained through selecting physical properties of the resin used as a binder, a solvent suitable therefor and a dispersing method and dispersing apparatus, and optimizing viscosity of a coating solution, dispersing power and dispersing time to thinly cover the surface of phosphor particles with a binder, thereby allowing the particles to closely approach to each other and forming a fine, soft and uniform network.
The above technique can be applied to the panel containing the stimulable phosphor.